The present invention relates to a method for the preparation of a shaped article of a vinyl chloride-based resin composition having improved surface properties. More particularly, the invention relates to a method for the preparation of a shaped article of a vinyl chloride-based resin composition having improved surface properties in respect of the insusceptibility to surface stain, weatherability, heat resistance, resistance against chemicals and oils, insusceptibility to blocking, decreased bleeding of plasticizers and others.
As is well known, vinyl chloride-based resins belong to one of the most important classes of general-purpose synthetic plastics by virtue of the versatility in respect of the rigidity of the shaped articles thereof ranging from rigid articles without formulation of a plasticizer to flexible articles prepared by compounding the resin with a substantial amount of a plasticizer. Besides the plasticizers used for controlling the flexibility or rigidity of the shaped articles, vinyl chloride-based resins are compounded with various kinds of other additives or compounding agents such as stabilizers, lubricants, antioxidants, chelating agents, anti-fogging agents, ultraviolet absorbers, fillers and so on depending on the intended application of the resin composition including films, sheets, artificial leathers, tubes, hoses, packings, covering materials and the like used in medical implements, wrapping materials of foods, insulation of electric wires, building materials and the like.
One of the serious problems almost always encountered in these shaped articles of a vinyl chloride-based resin compounded with the above mentioned additives is that the plasticizer and other additive ingredients contained therein migrate toward the surface of the article eventually to cause an undesirable phenomenon of bleeding or blooming in the lapse of time so that the shaped articles suffer decreased, beauty of the appearance, adverse influences on the chemical and physical properties of the shaped articles, durability and other disadvantages. In particular, such a phenomenon of bleeding or blooming of the additive ingredients on the surface is quite undesirable in the applications in the fields of medicine or foodstuff when the additive ingredient is toxic or physiologically unacceptable.
Various proposals and attempts of course have been made in the prior art in order to overcome the above mentioned problems including irradiation of the surface of the article with ionizing radiations or ultraviolet light and treatment of the surface wih a chemical agent. These prior art methods are of course effective to increase the heat resistance and solvent resistance and to impart hydrophilicity, insusceptibility to accumulation static electric charge and printability to the surface. These prior art methods, however, are rather ineffective when bleeding of the plasticizer and the like on the surface should be prevented or reduced with, on the other hand, some adverse effects on the advantageous properties inherent in the vinyl chloride-based resin.
Namely, the ionizing radiations usually have high energy so that the effect thereof is sometimes not limited to the very surface of the articles but the radiation penetrates more or less to the depth of the article to cause undesired alterations in the properties of the article due to, for example, formation of intermolecular crosslinks. When a shaped article of a vinyl chloride-based resin composition is irradiated with ultraviolet light, the surface of the article is sometimes not free from the problem of disadvantageous coloration. The chemical means in the prior art are also not free from the problems in respect of the corrosiveness of the treatment agent, poor adhesion of the agent to affect the durability of the effect by the treatment and so on.
A considerably effective method has recently been reported to improve the surface properties of a shaped article of a vinyl chloride-based resin composition that the surface of the shaped article is subjected to a treatment of exposure to an atmosphere of low temperature plasma so as to prevent bleeding of the plasticizer contained therein on the surface. The method of low temperature plasma treatment is indeed effective at least for a length of time immediately following the plasma treatment but the durability of the effect is far from satisfactory since the effect gradually disappears in the lapse of time when the treated article is used prolongedly, especially, in an outdoor condition so that it has been eagerly desired to develop a novel and effective method in the above mentioned object without the problems and disadvantages in the prior art methods.